Isotopes (from the Greek for "same place") of an element differ only in the number of neutrons in their nuclei. The number of protons and electrons for different isotopes of the same element is exactly the same and therefore the chemical properties of the isotopes are the same. Separation, then, depends on subtle differences in physical properties.
Isotope separation was performed on a laboratory scale before World War II prompted a massive exploration of possible methods. Since the War, there has been continued interest, prompted by the high capital and operating costs of the present methods.
It is convenient to divide the prior art into three classes:
The first class consists of those methods that do not ionize the atoms or molecules of the isotopes. This class includes the gaseous diffusion method, the principle of which is that molecules containing a heavier isotope will move less quickly through a membrane than will those containing a lighter one. As the difference in mass is small for the two species of uranium hexafluoride, for example, the amount of separation achieved by each step of the process is also small and many steps are required.
In contrast, separation methods that involve ionization feature large amounts of separation per step. The second class of methods involves the ionization of all isotopic species and separation of ions by mass. An example of this class is the electromagnetic apparatus, in which a combination of electric and magnetic fields produces a force on an ion that is directly related to its mass. This method is characterized by a high degree of separation and low limits on the amount of material that a device is capable of processing.
Methods in the last class are those in which one isotope is ionized (or excited) to a greater degree than the other (or others). This class is generally referred to as photochemical separation, whether the step of ion removal is done by physical means (such as deflecting the ions by a magnetic or electrostatic field), or by chemical means (such as an endothermic reaction that is not energetically available to an unexcited atom). This class has been a popular research area in recent years, with an emphasis on the use of lasers to excite or ionize one isotope in preference to others.
Different isotopes emit radiation from the same excited state at different frequencies, the difference in frequency being referred to as the isotope shift. The magnitude of an isotope shift will vary, depending on the isotope being compared, the temperature, the particular state involved, etc. When a particular isotopic shift is great enough so the ranges of the wavelengths emitted by the respective isotopes do not overlap, it is possible to excite only one of two or more isotopic species.
The standard approach of the photochemical art has been to use a laser with a frequency band narrower than the isotopic shift so that radiation that is tuned to the frequency emitted by one isotope will not interact with another isotope.
A succint statement is contained in a report, Photochemical Isotope Separation as Applied to Uranium by R. L. Farrar, Jr. and I. F. Smith, K-L-3054, Oak Ridge Gaseous Diffusion Plant, Oak Ridge, Tennessee (1972) which states
"The principal requirements for the successful application of the process involve three considerations. First, an isotopic shift must exist in the absorption spectra of the element or one of its compounds so that only one isotopic species of the process material can be excited or activated. Second, a light souce or an energy source must be obtained which is capable of emitting light at a very narrow wavelength, sufficiently narrow and at the proper frequency to excite only one of the desired group of isotopic species. Third, a chemical reaction or a physical process must be found in which only the exited species takes part. This process could, in principle, separate a middle isotope as easily as an end one, in a group of three or more isotopic species."
In contrast to the prior photochemical art, the present invention does not employ an external source of radiation to excite and ionize one isotope, as is stated in the second requirement given immediately above.
The prior art closest in structure to the present work ("Tests on the Separation of Isotope Molecules in a DC-Glow Discharge", W. Groth, P. Harteck, Naturwissenschaften, 22, p. 390, (1939)) achieved separation of hydrogen and deuterium in an electron discharge in an apparatus of different structure, by means of a different mechanism. In contrast to the present work, they used a hydrogen-deuterium mixture, and ionized both hydrogen and deuterium, the separation of which was accomplished as a result of the slower mobility of hydrogen molecular ions.
They attemtped to separate other elements (neon and xenon) but were unable to observe any separation, presumably because other elements have less difference in mobility between isotopes. This portion of their work teaches away from the present invention.